Synchronization system for television



Jan. 7, 1941. P. M. G. TOULON SYNCHRONIZATION SYSTEM FOR TELEVISION Filed July 5, 1937 ATTQRNEYS frequencies.

Patented Jan. 7, 1941 tam 1' warns;

' SYNCHBONIZATION SYSTEM Fon. ma- 7 VISION A Pierre Marie Gabriel Toulon, Puteaux, France Application July '3, 1937, Serial No. 151,807 7 In France July 8, 1936 v 4 Claims. (01.25046) r a.

'voltage resulting therefrom in the resistance is This invention relates to a new process to assure the synchronism of a relaxation oscillatory sys- 'tem by means of electric signals coming from an.

outside station. It concerns more particularly the synchronization of television receiving sta-* tions comprising a relaxation device controlled by the synchronization signals coming from a transmitting station.

One of the most important applications of the invention is that of assuring the synchronism between a transmitting station and the electronic beam or a Braun tube at the receiving station.

Present-day Braun tube receivers generally comprise two beam deflecting devices, each consisting of a condenser which is charged progressively, and a relay of the extreme vacuum or gas type through which the condenser suddenly discharges when this relay receives the synchronization signals coming from the transmitting station. This system has the drawback that if one of the signals fails or if a parasitic impulse releases the relay out of time, then the image becomes defective.

The present; invention is based on an entirely different process which consists in realizing locally a relaxation device having a frequency very near that of the frequency of the lines or of the images of the transmitting station, and which is completely controlled and compelled to remain in coincidence with that of the transmitting station, by the continuous measurement of thedifference in phase between these two Contrary to what has been done up to the present time, the signal received is not used to release the local relaxation device, but the latter operates independently of the transmission of the synchronization signals and its frequency is controlled by a device which automatically compensates for the lag between the synchronization signals and the oscillations of the relaxation device.

In accordance with the invention, the very short synchronization impulses are transformed into signals the shape of which resembles that of the oscillations of the local relaxation device, and the difference in phase between the two currents of the same form is measured. The result of this measure is most frequently that of causing a drop in voltage across a resistance which is a function of the lag, and this variation in voltage is utilized in order to correct the frequency of the local relaxation device. The arrival of a parasitic impulse or the absence of the synchronization signal has practically no efiect on the quality of the image, because the variation in the drop of too small to modify in an appreciable manner the cycle of the relaxation device; in other words, the framing of the image suffers practically no modification.

The invention will be better understood by reference to the drawing, which illustrates by way of example a special manner of execution of complete ,control of the frequency of a Thyratron" relaxation device which feeds a Braun tube by means of the synchronization pulses of a standard Hertzian wave television transmitting station and in which:

Figs. 1 and 2 represent, as a function of time, the form of the currents in the difierent parts of the assembly; and

Fig. 3 shows the basic diagram of a synchronizing system in accordance with the invention. In Fig. 1, i is the form of the synchronizing signals after selection. By means of a circuit having a certain time constant, which is introduced in accordance with the invention, these signals are transformed into a saw-tooth curve 2. The curve 3 in full lines represents the voltage of the same form supplied by the local relaxation device, which is assumed to be almost coincidental in phase with the television transmitting station. The curve 4 in full lines shows likewise the Voltage of this relaxation device in the case where a large phase difierence exists between the local relaxation device and the transmitting station. In this figure there can be seen in dot and dash lines the inverse curve of the voltages 3 and 4.

In accordance with the invention the phase difference between the curve 2 and one of the curves 3 or 4 is measured. This measurement is obtained by taking the difference between the two voltages and by rectifying the effective current which isgenerated in a transformer. This operation consists in adding the ordinates of the curve 2 with the curves in dot and dash lines representing the inverse of either curve 3. or curve 4.,

The principle of this measurement appears clearly if reference is made to Fig. 2, where it can be seen that it is possible to obtain very easily a voltage whose efiective value depends on the phase cate the average rectified voltage which corresponds respectively to the alternating voltages and 6. greatly differ in the two cases and, therefore, depend on the lag or displacement between the curves 2 and 4 or 2 and 3 of Fig. 1. When the voltages supplied by the synchronizing pulses and by the local relaxation device are essentially in phase, the average value of the corresponding rectified voltage is very weak, as indicated by line I. On the contrary, if the lag is appreciable, the rectified voltage is considerable, as shown by line I. This rectified voltage is used to vary the frequency of the local relaxationdevice in such a manner as to make it completely subservient to the frequency of the synchronizing signals.

If the relaxation device consists of a gas-filled tube such as a Thyratron associated with a fixed condenser and a resistance, the relaxation frequency may be varied within rather wide limits by changing the value of the polarization of the Thyratron grid. As this polarization increases, the frequency of the oscillations decreases. It can easily be understood that it is possible to realize automatic control if the lag is initially reduced to a very low value. If the lag tends to increase, the frequency of oscillation of local relaxation device tends to vary in such a direction that it compensates this lag due to the modification of the polarization of the grid.

Fig. 3 shows the details of a special assembly utilizing the proposed control system in the particular case in which a Thyratron is used in the relaxation device. In this figure, 8 designates the cathode of the Thyratron? and In its plate. The condenser ll, charged through the resistance l2 by the continuous voltage source 20, constitutes with this resistance and with the Thyratron the local relaxation device which serves to displace the cathodic beam in the Braun tube 22. 23 designates the radio receiving set which receives the image impulses and the synchronization impulses; the latter are selected by the tube 24 in accordance with their amplitude. Due to a resistance l5 and a condenser If! these impulses are transformed into a saw-tooth curve. The voltage which corresponds to the difference between the drops in voltage in the resistances l2 and I5 is applied to the primary winding 16 of a transformer. Thus the amplitude of the alter nating current corresponds to the difference between the drops in voltage in the resistances l2 and i5. The secondary winding ll of this transformer charges, through a rectifier l8 consisting for instance of a copper-oxide disc, a condenser l4 which slowly discharges through the resistance iii. The grid of the Thyratron 9 is connected through a resistance 9' with the terminal of this condenser [4. The average potential on grid 9 thus varies with the voltage between the terminals of the circuit I3-l4 which has a certain time constant; in other words the average potential on the grid of the Thyratron is a function of the lag between the oscillations of the local relaxation device and the synchronization signals. This average potential defines the frequency of the relaxation device. 1

Under normal conditions the Thyratron discharges slightly before the arrival of the synchronizing pulse. There results from this a certain effective value of the current in the transformer I6-l| and a certain value of the rectified current and therefore a certain polarization of the grid 9. This value corresponds exactly to that for which the frequency of the relaxation These values of the rectified voltage.

device is equal to the frequency of the synchronization signals. The system is controlled for the reason that if the local relaxation device tends to accelerate for any reason whatsoever there is an increase in the phase difference between the saw-tooth currents in the resistances i2 and H5. The current in the transformer lG-ll also increases and also the polarization of the grid 9. This has the desired effect of reducing the frequency of the relaxation device. Vice versa, if the relaxation device tends to slow down, the phase difference decreases and likewise the average polarization of the grid 9. The effect of this is to increase the frequency of the relaxation device. The relaxation device therefore remains in this manner in synchronism and in phase with the synchronization signals.

The advantage of this device over former systems lies in the fact that the synchronization signals may be absent either on account of fading or on account of parasitic disturbances or else additional disturbing signals may enter, without causing the local relaxation device, however, to cease to operate and consequently without causing the sweeping or scanning of the lines or images of the Braun tube 22 to cease to exist.

On the other hand, the changes which may appear in the frequency of the sweeping or scanning on account of these different disturbances are very slow and very gradual, so that the lateral displacements of the image which may originate on account of the lack of synchronization signals or on account of the presence of disturbing signals are of a very slight amplitude and there do not appear any longer on the screen of the Braun tube completely desynchronized strips which completely blur the image, as in the case of present-day apparatus.

Having thus described my invention, what I claim is:

1. In a television system, a receiver including a relaxation device having a saw-tooth output waveform, a source of synchronizing signals external to said receiver, means for transforming said synchronizing signals to have a. waveform substantially the same as the output waveform of said relaxation device, and means for varying the frequency of said relaxation device in accordance with the phase difference between the output of said relaxation device and said transformed synchronizing signals.

2. In a television system, a relaxation device having a saw-tooth output waveform and including a gas-filled tube having a grid, a source of synchronizing signals, means for transforming said synchronizing signals to have a waveform substantially the same as the output waveform of said relaxation device, means for producing a direct-current voltage the amplitude of which is a function of the phase difference between the output of said relaxation device and said transformed synchronizing signals, and means for applying said direct-current voltage to said grid.

3. In a television system, a receiver including a relaxation device having a saw-tooth output waveform, a source of synchronizing signals external to said receiver, means for transforming said synchronizing signals to have a waveform substantially the same as the output waveform of said relaxation device, means for producing a potential which is a function of the phase difference between the output of said relaxation device and said transformer synchronizing signais, and means for utilizing said notential to a direct-current voltage the amplitude of vvhich vary the'trequency oi. said relaxation device. is a function of the phase diiierence between the 4. In a television system, a relaxation device output of said relaxation device and. said-transhaving a saw-tooth output waveform, a sourceiot formed synchronizing signals, and means for synchronizing signals, means for transforming utilizing said direct-current voltage tovary the 5 said synchronizing signalstohave a'waveform frequency of said relaxation device. substantially the same as the output waveform of said relaxation device, means for producing PIERRE MARIE GABRIEL TOULON. 

